1. Field of the Invention
The present invention relates to a light irradiating device, and more particularly, to a large scale light irradiating device which is used in a photo-alignment process of a multi-domain liquid crystal display device.
2. Description of the Related Art
Generally, a multi-domain liquid crystal display device comprises upper and lower substrates placed to face each other with a specific gap provided by a spacer, and a liquid crystal layer is formed between the upper and lower substrates. The upper and lower substrates respectively have an electrode of specific patterns on one side, and an alignment layer, which determines the alignment of the liquid crystal, is formed on the electrodes.
A rubbing method or a photo-alignment method or the like is used as an alignment method treating the alignment layer. The rubbing method is composed of coating an alignment material, e.g., polyamide (PI), on a substrate, and then rubbing the substrate mechanically with a rubbing cloth. From the above process, an alignment direction of liquid crystal molecules on the substrate is obtained. This method enables an LCD to be made on a large scale and to be treated rapidly.
In the rubbing process described above, however, the alignment of liquid crystal molecules is not uniform because the figure of microgrooves formed in the alignment layer varies by rubbing strength. And this causes light scattering and random phase distortion. Also, dust particles and electrostatic discharge produced by rubbing causes a decrease in yield, and the repeated photolithography process for realizing multi-domain by pixel division effects the reliability and stability of the alignment layer.
On the other hand, the photo-alignment method determines the pretilt of liquid crystal (LC) by irradiating ultraviolet light over a substrate having a photo-alignment layer. Compared with the rubbing method, neither electrostatic discharge nor dust particles are produced, so the manufacturing yield is maintained. Especially, by this method, realization of a wide-viewing angle LCD by pixel division is made possible.
The above light irradiating device used in a photo-alignment process is disclosed in JP-A-10-90684 (published Apr. 10, 1998) and JP-A-10-161126 (published Jun. 19, 1998).
FIG. 1 is a configuration of the conventional light irradiating device in JP-A-10-90684, which relates to a polarized-light irradiating device used for irradiating polarized light on the alignment layer of LCD in a photo-alignment process for the alignment layer.
The light-containing ultraviolet light emitted from a light source 1 is concentrated at a condensing mirror 2, reflected at a first reflective mirror 3, and then goes through a condensing lens 5. The light from the condensing lens 5 via a shutter 4 is reflected at a second reflective mirror 6, made to be in parallel by a collimating lens 7, and then passes to a polarizer 8. The polarizer comprises a plurality of glass plates 8a which are placed in parallel at a fixed interval, and placed with Brewster's angle with respect to incident light. It reflects most vertically polarized light and transmits horizontally polarized light. Horizontally polarized light from the polarizer 8 is irradiated on substrate 35 by way of mask 13.
In the light irradiating device which has the configuration mentioned above, the polarization ratio (s/p, wherein s is vertically polarized light, p: horizontally polarized light) is set to be less than 0.1. In order to apply photo alignment to an alignment layer of the LCD, polarized lights having the same polarizing direction are irradiated. And this device is excellent in transmittance, wavelength dependency, durability and life duration. But, to produce a large scale LCD, the glass plate 8a of the polarizer 8 has to be large. And the range of polarization ratio of this device is inappropriate to conduct the photoalignment process effectively.
FIG. 2 is a schematic diagram of the configuration of a conventional light irradiating device, described in JP-A-10-161126, where an exposure device comprises light source 1, condensing mirror 2, collimating lens 7, homogenizer 19, condensing lens 5, one or more reflective mirror(s) 3, 6 leading the light from the light source 1 to a substrate 35. And at least one reflective mirror therein comprises a reflective diffraction grating reflecting mainly the first polarized light.
Because the first reflective mirror 3 is generally smaller in scale than the second reflective mirror 6, when the first reflective mirror 3 is composed of a diffraction grating, the diffraction grating on a small scale can be used. Since the diffraction grating is located before the homogenizer 19, light through the homogenizer is prevented from being affected by the diffraction grating.
By the configuration described above, the light irradiating device can irradiate polarized light on a large scale at once by irradiating reflected light from a reflective diffraction grating reflecting mainly the first polarized light. The polarization characteristic of the diffraction grating in the light irradiating device, however, depends greatly on wavelength.
The above prior art is inappropriate for the actual photo-alignment process because it is focused on annexing polarizing devices to the conventional light irradiating devices to obtain polarized light.